1. Field of the Invention
The present invention relates to the field of circulating fluid bed boilers (CFB's) such as those used in the production of steam for industrial process requirements and/or electric power generation and, in particular, to a reversible, wear-resistant ash screw cooler section which provides extended service life.
2. Description of the Related Art
A fluidized bed boiler, as used in the context of industrial processes and/or utility power generation, is similar to a water-filled container. Instead of water, however, the material in the bed is a congregation of granular solid particles (fuel and sorbent, such as limestone) in a state of mobile suspension as a result of the upward flow of air or gas. Initially, the bed's individual solid particles or granules are motionless and supported by contact with each other. The material as a whole rests on what is known as a distributor plate or grid plate, which has openings in it to permit the upward flow of air or gas therethrough, as well as drains for waste material.
As the rate of upward air or gas flow through the distributor plate is increased, the individual particles or granules within the bed will begin to move. In a non-circulating or bubbling bed type of fluidized bed boiler, the flow rate is increased until the fluidization point is reached. The term fluidization point denotes that point where the air or gas flow upward through the bed expands the solid bed sufficiently to allow the granules to move within the bed. That is, this is a condition where the individual particles or granules of solid are suspended by the air or gaseous fluid passing through them at a velocity sufficient to cause them to be disengaged somewhat from each other. The air or gas flow upward through the bed determines the amount and size of voids between the particles, and the overall level of the fluidized bed is determined by the amount of particles, that is, fuel, inerts, etc. in the bed as expanded/fluidized by the air or gas flow. The upward flow rate of air or gas is maintained at a level to maintain the bed at the fluidization point, and to minimize any transport of particles out of the bed into the upper furnace area.
In contrast, a circulating fluid bed boiler is maintained with upward air or gas flows which exceed the fluidization point, which results in particles being carried out of the bed into the upper furnace area. FIG. 1 is a sectional side view of one such CFB boiler design of The Babcock & Wilcox Company, the Ebensburg Power unit located in Ebensburg, Pa. This boiler burns waste bituminous coal to produce, at maximum continuous rating (MCR), 465,000 lb/hr main steam flow at 955.degree. F. and 1550 psig. The high fuel ash content at this unit (40% design maximum) produces furnace circulating solids with a much higher percentage of ash than with a typical bituminous coal, and leads to severe erosion problems.
As shown therein, the boiler has a furnace enclosure that is top-supported from structural steel and is constructed of gas-tight membrane walls. Coal and limestone (for sulfur dioxide removal) are fed through the furnace front wall using four injection screws. Combustion air from a single forced draft fan is split downstream of the air heater into primary and overfire air. The primary air is introduced into the bed through the furnace floor from a compartmented windbox, while the overfire air enters the lower portion of the furnace through different size nozzles at two elevations on the front and rear walls. Adjustment of these two air streams provides proper air distribution and air-gas mixing to achieve fuel burnout in the furnace.
The solids handling system for the Ebensburg unit is shown schematically in FIG. 2. Located above the furnace I is the primary solids collector; an impact-type separator consisting of a staggered array of U-shaped elements (U-beams) 9, 10 hung from the boiler roof forming a labyrinth passage for gas and solids. The first two rows of the primary collector, i.e., the in-furnace U-beams 9, are located just upstream of the furnace outlet where they discharge collected material directly into the furnace I along the rear wall. The solids collected by the other rows 10 of the primary collector are discharged into a particle storage hopper and returned to the lower furnace through four non-mechanical L-valves 11.
The total solids inventory in the bed is controlled by removing the bed material through four bed drain pipes 2 installed in the furnace floor and water-cooled screw coolers 3. The screw coolers 3 take ash from the fluidized bed at temperatures in the range of 1600.degree. F. and cool it to approximately 450.degree. F. The bottom ash is routed through a rotary valve 7 to a screening device 4 where the fine material is separated and pneumatically conveyed in dense phase transport 5 to a bed drain injection bin 6 located on the boiler front wall.
The solids flow control by L-valves 11 allows exchanging of the solids inventory between the furnace I and particle storage hopper for the furnace process control. The two-stage primary collector arrangement 9, 10 reduces the amount of external recycle needed to maintain the furnace inventory. Solids escaping the primary collector 9, 10 into the convection pass enter the multiclone dust collector (MDC) 13 located between the economizer and the air heater 17. Solids collected by the MDC are recycled to the lower furnace via a dilute phase pneumatic recycle system 14, 15. Excessive solids are purged from the MDC hopper 13, while solids leaving the air heater 17 are collected in a baghouse 18.
Cooling the ash drained from the bed has required fluid bed ash coolers, ash screw coolers, or both. One type of ash screw cooler 20 is known as a HOLO-FLITE.sup.R ash screw cooler, manufactured by Denver Equipment Company, Colorado Springs, Colo., and a sketch of same is shown in FIG. 3. The drive 22 for the ash screw cooler 20 is generally a variable speed type, and can be controlled by the plant control system (not shown) using a signal developed from pressure drop across the fluidized bed. Primary cooling of the ash is achieved via the screw flites 26 themselves, which are hollow and circulate cooling water 28 therethrough. Typical cooling water pressures are 150 or 250 psig on the screw and 30 or 50 psig on the trough jacket 30. Because of the high ash temperatures, the entire feed end 24 of the ash screw cooler 20 must also be cooled, including the end plate 32, the first section of the trough cover 34, and feed nozzle 36.
The Ebensburg unit has experienced extreme erosion in the bed drain ash screw coolers. The erosion has been so severe that the inner liners of the troughs failed by wearing through after only a short period of time, resulting in water leaking from the trough. A replaceable end trough section, having an inner liner of 1/2 inch thick stainless steel, was retrofitted to these ash screw coolers, but failed to significantly reduce the time to failure even though the use of stainless steel liners in previous "high wear" situations had been successful.
It is thus apparent that a solution to this erosion problem concerning the ash screw coolers is required.